The present invention relates to an electrostatic chuck for holding a substrate during processing of the substrate in a process chamber.
Electrostatic chucks are used to hold substrates in various applications, including for example, holding a silicon wafer in a process chamber during semiconductor fabrication. A typical electrostatic chuck comprises an electrode covered by dielectric material, for example, a ceramic such as aluminum oxide or aluminum nitride. When the electrode is electrically biased with respect to the substrate, an attractive electrostatic force is generated that holds the substrate to the chuck. In monopolar electrode chucks, an electrically charged plasma above the substrate provides electrostatic charge to the substrate, and in bipolar electrode chucks, bipolar electrodes are electrically biased relative to one another to provide the electrostatic attractive force.
The electrostatic chuck is attached to a support or pedestal in the process chamber through which an electrical connector extends to connect the electrode of the chuck to a voltage terminal in the process chamber. Typically, the support is made of metal to serve as a process electrode of a plasma generator that capacitively couples RF energy to process gas in the chamber to form a plasma. In conventional chucks, an insulating plug is provided around the electrical connector to electrically insulate the connector from the metal support. However, during plasma processing the electrostatic chuck and support are heated, and the differing coefficients of thermal expansion between the electrostatic chuck and the support often causes a failure in electrical connection or cause the electrical connector to crack or break. This is particularly a problem for electrostatic chucks having dielectric members made from a material having a high thermal expansion coefficient, such as aluminum oxide, or made from a material having a thermal expansion coefficient that widely differs from the expansion coefficient of the support. Thus it is desirable to have an electrical connector that allows relative movement between the electrostatic chuck and the support without interrupting or shorting the electrical connection.
Another problem with the electrical connector in conventional electrostatic chucks occurs because the interface between the chuck and the support does not form a gas tight seal. Process gas permeates into the space between the chuck and the support and reaches and surrounds the electrical connector during operation of the chuck. The chemically reactive process gas erodes the electrical connector or its surrounding insulator. For example, CF.sub.4 gas rapidly erodes ceramic oxides, and O.sub.2 gas rapidly erodes polymer materials. In addition, the high voltage applied to the electrode of the chuck that passes through the electrical connector couples to the gas surrounding the electrical connector and causes arcing and plasma glow discharge in the process gas. The arcing and plasma glow discharges provide electrically conducting pathways that disrupt the process plasma by leaking energy from the plasma or damaging surrounding electrical connector components. It is desirable to have an electrical connector that reduces, or entirely eliminates plasma glow discharge and arcing of the gas around the electrical connector.
A further problem arises when gas, such as air or nitrogen, that is trapped in spaces within the electrical connector, slowly leaks out when the process chamber is pumped down during a low pressure vacuum operation. The resultant "virtual leak" of slowly escaping gas substantially increases the time taken to pump out the process chamber. Moreover, even when the process is performed at relatively low vacuum (high pressure), for example above 40 mTorr, the trapped gas should be removed before processing a substrate, because the leaking gas changes the composition of the process gas and disrupts processing uniformity across the substrate surface, especially when the trapped gas contains water vapor.
Thus there is a need for an electrostatic chuck that has an electrical connector with an enclosed substantially gas sealed environment. There is a further need for an electrostatic chuck that reduces plasma glow discharge or arcing of the gas around the electrical connector of the chuck. It is further desirable for the chuck to have an electrical connector that allows thermal expansion of surrounding structures without damaging or breaking the electrical connection to the electrode of the chuck. There is a further need for an electrostatic chuck having an electrical connector which will not rapidly degrade in erosive process gas environments.